1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for selecting a cell in a RRC connection reestablishment procedure.
2. Discussion of the Related Art
A basic purpose of selecting a cell is to register user equipment (UE) in a network and receive a service from a cell. If the intensity and the quality of a signal between a UE and a cell become inferior due to the mobility of the UE, the UE reselects another cell in order to maintain data transmission quality.
The above-mentioned cell selection process is divided into two categories such as following.
First one is an initial cell selection, that the UE does not have information about radio channel. Therefore, the UE may retrieve every radio channel in order to select a cell, and then may select a cell corresponding to a radio channel of which a signal quality is strongest among the retrieved radio channels.
Second one is a stored information cell selection, that the UE uses stored information. The stored information is already stored in the UE or is broadcasted by the cell. In this case, the UE can perform the cell selection process more quickly than the first case, since the UE already has the information about the radio channels or can perceive instantly the information about the radio channels through the broadcast information.
An equation 1 shows the cell selection criterion of LTE (Long Term Evolution) system described in 3GPP TS 36.304.Srxlev=Qrxlevmeas−(Qrxlevmin+Qrxlevminoffset)−Pcompensation>0  [Equation 1]
Parameters using the equation 1 are shown in table 1.
TABLE 1QrxlevmeasMeasured cell RX level value (RSRP).QrxlevminMinimum required RX level in the cell (dBm)QrxlevminoffsetOffset to the signaled QrxlevminPcompensationmax(PEMAX − PUMAX, 0) (dB)PEMAXMaximum TX power level an UE may use whentransmitting on the uplink in the cell (dBm)PUMAXMaximum RF output power of the UE (dBm)
The UE may receive the parameters shown in table 2 through SI (System Information), and then may perform the cell selection process using the cell selection criterion shown in equation 1.
The SI includes basic information necessary for accessing a network. Accordingly, the conventional UE should receive the SI before accessing the cell and should have the newest SI. Since the SI must be known to the UE in the cell, the cell may broadcast the SI periodically.
The SI is divided into MIB (Master Information Block), SB (Scheduling Block), SIB (System Information Block), and so on. MIB informs the UE of physical information of the cell, such as bandwidth. For example, one SIB includes only a neighbor cell list (NCL), and other SIB includes only information about uplink radio channel which is used by the UE. Further, SB informs the UE of information about transmitting SIB, such as transmitting period.
If the intensity and the quality of a signal between a UE and a cell become inferior due to the mobility of the UE, the UE reselects another cell in order to maintain data transmission quality.
On the other hand, after UE selects certain cell through cell selection process, the intensity and the quality of a signal between the UE and the cell become inferior due to the mobility of the UE. If the quality of the cell becomes inferior, the UE may select other cell which provides the UE with better quality signal. This procedure is referred to as a cell reselection process.
As a view of the quality of radio signal, a basic object of the cell reselection process is selecting a cell that provides the UE with best quality signal. Besides of the view of the quality of radio signal, a network may decide a priority of frequency, and then may inform the UE of the priority. The UE may consider the priority prior to the cell selection criterion.
Conventionally, methods for reselecting a cell differ according to parameters associated with a radio access technology (RAT) and a frequency characteristic of the cell participating in the reselection of the cell. The conventional methods are divided such as following table 2.
TABLE 2Intra-frequencyA UE reselects a cell having the same centercell reselectionfrequency and RAT as a serving cellInter-frequencyA UE reselects a cell having the same RAT ascell reselectiona serving cell and a center frequency differentfrom the serving cellInter-RAT cellA UE reselects a cell using a RAT different fromreselectiona RAT which is being used by a serving cell
FIG. 1 shows the conventional operation of a UE in an idle mode when the UE is powered on.
Referring to FIG. 1, when the UE is powered on, the UE automatically or manually selects a public land mobile network (PLMN) from which a service is desired to be received, and a RAT for communication (S110). The PLMN and RAT information may be selected by the user of the UE or by the PLMN. Alternatively, the PLMN and the RAT information stored in a universal subscriber identity module (USIM) may be used.
In this case, UE may measure a radio signal received from a cell (S130), which is a reference signal or a pilot signal, and calculate a cell quality value using the characteristic of a physical signal associated with the intensity of a signal or a signal-to-noise interference ratio.
Thereafter, the UE performs a cell selection process of selecting a cell having a highest cell quality value among cells of which the cell quality value is larger than a reference value (S120). The reference value indicates a value which is defined in a system in order to ensure the quality of a physical signal in transmission and reception of data. Accordingly, the reference value may be changed according to the applied RATs, and may be applied by the equation 1 in LTE system
Thereafter, the UE receives system information (SI) that is periodically transmitted by the base station. The UE registers its information (e.g., international mobile subscriber identity (IMSI)) in a network in order to receive a service (e.g., paging) from the network (S150). The UE does not register its information whenever the cell is selected. Instead, the UE registers its information in the network only if needed. For example, the UE registers its information in the network if network information (e.g., tracking area identity (TAI)) received from the SI is different from the network information about which the UE knows (S140 and S170).
If the cell quality value of the serving cell is lower than that of a neighbor cell, the UE reselects one of other cells providing with better signal characteristics than the cell of the base station which the UE accesses. In order to distinguish this procedure from the cell selection process of the step S120, this procedure (S160) is called a cell reselection process. In the cell reselection process, a time limit condition (ex. Cell select timer) is configured in order to prevent the UE reselecting a cell frequently according to a signal characteristic.
On the other hand, if the conventional cell selection process is directly applied to the RRC connection reestablishment procedure, the UE always transmits a RRC connection reestablishment request message to the selected cell regardless of a type of the selected cell. That is, though the selected cell can not provide the UE with normal services, the UE transmits the RRC connection reestablishment request message to the selected cell. As consequently, the cell may transmit a RRC connection reestablishment reject message to the UE. In this case, it is a problem that the UE has to perform all procedures for being provided with the services by the cell, in order to maintain the services.
Further, in the conventional cell selection process, the UE considers cells that use all RAT (Radio Access Technology) supported by the UE. Therefore, if the conventional cell selection process is directly applied to the RRC connection reestablishment procedure, the UE can select the cell using not E-UTRA but other RAT. However, since the cell using other RAT does not have a context and a configuration of the UE, the UE recognizes the RRC connection reestablishment procedure as a failure. Then, it is a problem that the UE may leave from the RRC_CONNECTED state to the RRC_IDLE state though there is sufficient time for retrieving the cell using E-UTRA.
Further, if the conventional cell selection process is directly applied to the RRC connection reestablishment procedure, the continuity of the RRC connection can not be ensured.